Generally employed suspending agents for suspension polymerization are in the form of a slurry comprising water and inorganic suspending agent particles. In most of such suspending agents, the inorganic suspending agent particles have a weight-average particle diameter of about from 2 to 3 pm and have a particle size distribution in which the particle diameter at weight-cumulative 95%, from the minimum particle size, is about from 20 to 30 .mu.m.
However, the inorganic suspending agent particles contained in those suspending agent slurries are apt to aggregate into coarse particles with the lapse of time. Use of a suspending agent slurry containing coarse agglomerates of inorganic suspending agent particles in conducting suspension polymerization poses a problem that a polymer is obtained as coarse beads or coagulates. Thus, the conventional suspending agents are less apt to retain their ability over long.
Even when a conventional suspending agent slurry in which the inorganic suspending agent particles have not agglomerated is used to conduct suspension polymerization, there are problems that the polymer beads obtained tend to have a broad bead size distribution, and that an attempt to obtain polymer beads having a given bead diameter range results in a reduced yield.
On the other hand, an attempt has been made to obtain a narrower bead size distribution by adding various salts to an aqueous dispersion medium for use in polymerization or by regulating the hydrogen ion concentration of an aqueous dispersion medium at the initial stage of polymerization. However, use of these techniques poses a problem that the suspending agent particles, especially when having a small particle diameter, are apt to agglomerate into coarse particles which apparently have a far larger diameter than the primary particles of the suspending agent, whereby the polymer beads finally obtained have an increased bead diameter.
In general, it is preferred to finally obtain polymer beads having a bead diameter around 1 mm in high yield with a smaller suspending agent amount. Attempts are hence being made to reduce the particle diameter of suspending agent particles.
For example, a technique of synthesizing a specific inorganic suspending agent and a technique of grinding an inorganic suspending agent have been proposed (see JP-A-4-309504). (The term "JP-A" as used herein means an "unexamined published Japanese patent application.") However, the former technique has a drawback that a special raw material is necessary. The latter technique is capable of yielding inorganic suspending agent particles having a reduced particle size. However, not only the reduced particle size is not sufficiently small, but also the heat generated by friction during grinding is apt to cause an increase in slurry viscosity and thermal agglomeration of suspending agent particles. Consequently, it is difficult with the latter technique to obtain a suspending agent having a desired particle diameter, and the slurry obtained has poor handleability due to its viscosity increase.
To mitigate the above-described problems, the grinding of a suspending agent should be performed in an extremely low suspending agent concentration, i.e., about 10% by weight or below. Although this technique is effective in mitigating the problems concerning the generation of heat in the suspending agent slurry and the viscosity increase caused by thermal agglomeration, industrial use thereof is still disadvantageous because large-scale treatment using this technique requires much time due to the low grinding efficiency thereof.